In general, in a wiper device to be mounted on a vehicle, a wiper motor is used as a driving source for periodically swinging a wiper arm (see for example Japanese Patent Application Laid-Open Publication No. 2009-189093). This wiper motor has: a motor unit having brushes and an armature; and a speed reduction mechanism unit for reducing the speed of rotation of the armature of the motor unit. The speed reduction mechanism unit is provided with a control device for controlling the rotation of the armature, thereby controlling a position and a speed of the wiper arm. In a wiper motor which can rotate its armature in a forward direction and a reverse direction by using a two-pole magnet and two brushes, since it is necessary to increase the thickness of the yoke on the basis of magnetic flux of the motor unit, there is a possibility of increasing the weight of the wiper motor. For this reason, its inertia is increased. Therefore, there is a possibility of deteriorating the control accuracy of the wiper.